Device And Method For Line Control Of An Energy Beam

ABSTRACT

The invention relates to the field of line control of a beam, and especially to a device comprising a plurality of ionisation chambers, enabling the measurement of the dose deposited by an ionising beam and the field of said beam. At least one ionisation chamber is formed from support films having a thickness less than or equal to 100 nm.

TECHNICAL FIELD

The present invention relates to the field of online beam monitoring.More particularly, the present invention concerns a device comprisingseveral ionisation chambers allowing the measurement of the dosedeposited by an ionising beam and the field of this beam.

TECHNOLOGICAL BACKGROUND

Hadron-therapy is a branch of radiotherapy allowing the delivery withprecision of a dose onto a target volume, a tumour, whilst preservingthe surrounding healthy tissues. Hadron-therapy apparatus comprises anaccelerator producing a beam of charged particles, means fortransporting the beam and a radiation unit. The radiation unit deliversa dose distribution to the target volume and generally comprises meansfor monitoring the delivered dose. Two major modes for delivering beamsof particles are used in hadron-therapy: one first delivery modecomprises so-called passive beam scattering techniques and a second moreelaborate treatment mode comprises dynamic beam scanning techniques.

The passive scattering methods have recourse to an energy degraderadjusting the pathway of the particles as far as a maximum depth pointof the region to be irradiated. The energy degrader is also used incombination with a range shifter wheel, a compensator and apatient-specific collimator allowing a dose distribution to be obtainedwhich best coincides with the target volume. One major defect of thistechnique is that the neighbouring healthy tissues located upstream andoutside the target volume may also be subjected to high beam doses. Inaddition, the need to use a compensator and a collimator specific to thepatient's tumour and to the angle of irradiation makes this procedurecomplicated and costly.

One mode for delivering a dynamic beam comprises the so-called “PBS”methods (Pencil Beam Scanning) in which a narrow beam of particlesoriented along axis z is scanned over a plane orthogonal to this axis zover the target volume by means of scanning magnets. By causing theenergy of the beam of particles to vary, different layers in the targetvolume can be successively irradiated. In this manner the radiation dosecan be delivered over the entirety of the target volume.

One first method of the so-called Pencil Beam Scanning technique is amethod called spot scanning. With this method, the irradiation of layersof target volume is obtained by delivering a prescribed beam dose todiscrete positions of this volume and interrupting the beam between eachchange of position.

Another Pencil Beam Scanning method is the so-called continuous scanningtechnique in which the beam is scanned continuously following apredefined pattern. During the scanning of a layer, the intensity of thebeam may vary at every instant so as to deliver a precise dose at theright place in the target volume, such as specified in the treatmentplan. In other more advanced beam delivery techniques, the scanning ratecan be adjusted instant by instant, so as to have an additional degreeof freedom to modulate the intensity of the beam.

With the PBS technique not only homogeneous distribution doses but alsonon-homogeneous doses can be delivered to a target volume. Typically, acombination of several treatments with beams from different directionsis necessary to produce a “tailored” radiation dose which maximizes thedose in the target volume whilst protecting neighbouring healthytissues. Although a three-dimensional dose distribution in the targetvolume resulting from radiation in a single direction may not beuniform, provision is made so that the contribution of each radiation inseveral directions produces a uniform dose in the target volume. Atreatment which delivers beams depositing non-homogeneous doses in whichintegration of each beam contribution allows a homogeneous dose to beobtained in a target volume is called Intensity Modulated ParticleTherapy (IMPT). The specification of the treatment is prepared byadvanced treatment planning systems using optimisation algorithms tospecify the number and the directions of beam treatments and theparticle intensities to be delivered to each point in each layer to beirradiated.

Another example of a dynamic technique is a radiation technique whichdiffers from PBS and is called a uniform scanning technique in which auniform dose is delivered to a target volume layer by layer, and inwhich the beam is continuously scanned assuming the form of a geometricpattern. The beam does not assume the shape of the contour of the targetvolume but is scanned over a predefined geometric surface area andlateral conformity is obtained by means of a collimator comprisingseveral plates or by means of a patient-specific aperture.

Through the complexity of these different techniques, the verificationof the dose sent to the patient is a crucial point. The calibration ofhadron-therapy apparatus is standardized and is made using a waterphantom which chiefly comprises a detector, generally an ionisationchamber or an array of pixels, which may or may not be able to be movedin a large container filled with water, the density and stopping powerof water being similar to those of human tissues. This calibration isperformed before treatment and the treatment plan is prepared on thebasis of this calibration.

Ionisation chambers are standard dosimetry detectors generally used inradiotherapy. An ionisation chamber comprises a polarisation electrodeseparated from a collecting electrode by a gap comprising a fluid of anytype.

There are several types of ionisation chambers such as so-calledcylindrical ionisation chambers and ionisation chambers comprisingparallel plates. Cylindrical ionisation chambers comprise a central oraxial electrode generally in the form of a very thin cylinder insulatedfrom a second electrode of hollow cylindrical shape or cap-shapedsurrounding the said central or axial electrode. Ionisation chamberscomprising parallel plates have a first plate supporting a polarisationelectrode, this first plate being separated from a second platecomprising one or more collecting electrodes located opposite thepolarisation electrode. The plates are separated by a gap comprising afluid of any type. The perimeter of each collecting or polarisationelectrode deposited on the plates is surrounded by an insulating resinitself surrounded by a guard electrode.

The fluid contained in the gap separating the collecting andpolarisation electrodes of an ionisation chamber used in dosimetry ismost often a gas. When an ionising beam passes through the ionisationchamber, the gas contained between the electrodes is ionised andion-electron pairs are formed. An electric field is generated byapplying a potential difference between the two electrodes of theionisation chamber. The presence of an electric field allows theseion-electron pairs to be separated causing them to drift onto therespective electrodes, thereby inducing a current at these electrodeswhich will be detected and measured.

During treatment, it is also essential to monitor the dose delivered tothe patient ensuring that it corresponds to the dose prescribed in thetreatment plan, for example by means of an ionisation chamber. It mustalso be possible to detect any deviation of the beam. The document: “Apixel chamber to monitor the beam performances in hadron therapy”, R.Bonin et al., Nucl. Instr. & Methods in Phys. Reas. A 519 (2004)674-686, describes an ionisation chamber comprising a cathode 25 μmthick composed of a mylar film on which aluminium has been deposited,and an anode composed of a Vetronite film of thickness 100 μm sandwichedbetween two films of copper each 35 μm thick. Using the PCB technique,the said anode is segmented into 32×32 pixels on one side and each pixelis connected by a via passing through the Vetronite film to a conductivetrace located on the other side of the anode. Each trace connects apixel to a signal measuring device. However, this pixel ionisationchamber has some shortcomings of which the first is mechanicalinstability. The distance between the two electrodes is defined by anexternal armature. Mechanical deformation or a microphonic effect mayaffect the distance between the two electrodes significantly, therebyaffecting the accuracy and precision of measurement. Another problemwith this device is its lack of <<transparency>> with respect to a beam.The non-negligible thickness of copper present on the anode induces beamscattering.

Document WO 2006126084 partly solves these problems by replacing thecopper layers forming each pixel by graphite layers. Also anintermediate layer pierced with holes surrounding each pixel is providedbetween the anode and the cathode thereby forming a plurality ofchambers. Attachment points fix the intermediate layer to the anode andcathode so as to allow air to pass and to stabilize the distance betweenthe anode and the cathode.

Nonetheless, this type of detector always induces angular andlongitudinal beam scattering, hence the need for the possible providingof a detector the most <<transparent>> possible, in other words whosewater equivalent thickness (WET) is minimal so as not to degrade theproperties of the beam.

In general, the water equivalent thickness of a portion of material m ofthickness l_(m) through which there passes a given beam of particles ofgiven energy is defined as the water thickness producing the same lossof energy of the beam as the portion of material m of thickness l_(m).The water equivalent thickness of a material m of portion l_(m) throughwhich an energy beam is passed is given by the following equation:

$\begin{matrix}{{WET}_{m} = {l_{m}\begin{matrix}\rho_{m} & \left( {\frac{1}{\rho}\frac{E}{x}} \right)_{m} \\\rho_{w} & \left( {\frac{1}{\rho}\frac{E}{x}} \right)_{water}\end{matrix}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Where:

p_(m) is the density of the material m, in g/cm³;

p_(w) is the density of water, in g/cm³;

l_(m) is the thickness of the material, in cm;

$\left( {\frac{1}{\rho}\frac{E}{x}} \right)_{m}$

is the stopping power of the material on the beam relative to thedensity of the material m, in MeV*cm2/g;

$\left( {\frac{1}{\rho}\frac{E}{x}} \right)_{water}$

is the stopping power of water on the beam relative to the density ofwater, in MeV*cm2/g.

Minimisation of the water equivalent thickness for an ionisation chambercan be obtained by reducing the thickness of the plates supporting theelectrodes and using materials for these plates of relatively low meanatomic weight. However, there is a limit thickness for theseelectrode-supporting plates below which several problems may arise.

One first problem to which consideration must be given is the increasein capacitance at the electrodes on the support film. Charge differencesthat are too high between the two sides of one same film may lead tobreakdown of the film. For a planar capacitor, capacitance is given by

$C = {ɛ_{0} \cdot ɛ_{r} \cdot \frac{A}{d}}$

where:

εO: vacuum permittivity;

εr: relative permittivity of the material;

A: area of the plate of the electrode;

d: thickness of the plate of the electrode.

A second problem is the presence of microphonic noise affecting thedistance between the electrodes and reducing the precision andexactitude of measurement.

Additionally, with a support plate of reduced thickness it becomesdifficult for a via to be passed through the plate to connect one ormore collecting or polarising surfaces with one or more conductivetraces without affecting the mechanical stability of the plate.

Document U.S. Pat. No. 6,011,265 describes a detector comprising asingle ionisation chamber comprising a plurality of support filmsarranged in parallel and separated from each other by a gap. Thedescribed ionisation chamber comprises:

-   -   a first support film comprising an electrode DE;    -   a second support film comprising a collecting electrode CE        composed of a plurality of elementary anodes;    -   one or two support films 10 contained between the said first and        second support films, the said support films 10 being made in an        insulating material and metallised on their two sides so as to        form a first metal cladding 11 and a second metal cladding 12,        the said metal clad films 10 comprising a plurality of        perforated holes, the whole forming an electron multiplier;    -   first polarisation means B1 for polarising the electrode D2        located on the first film;    -   second polarisation means B2 adapted to set up an electric        polarisation voltage between the said first metal cladding 11        and the said second metal cladding 12 so as to form, at each        hole, an electric field condensation region in which a condensed        electric field is generated, the said condensed electric field        functioning so as to generate an electron avalanche from said        photoelectron, considered to be a primary electron;    -   third polarisation means B3 adapted to create an electric        polarising voltage which is applied to the said collecting        electrode CE to allow the detection of the said electron        avalanche.

The detector described in U.S. Pat. No. 6,011,265 may also comprise asecond assembly of elementary anodes arranged on the second side of thesecond support film so as to form a two-dimensional detector. However,in hadron therapy techniques which notably use beam currents of highintensity, the beam monitoring devices used are ionisation chambersoperating at saturation for maximum efficacy of charge collection.Therefore, phenomena of charge recombination must be minimizedsubsequent to ionisation of the gas present inside an ionisationchamber, which may be detrimental to saturation of the chamber and henceto precision of measurement. As a result, it is not possible for thistype of beam to use an ionisation chamber in which there isamplification of the charges produced subsequent to ionisation of thegas, such as described in document U.S. Pat. No. 6,011,265.

Aims of the Invention

It is therefore necessary to be able to produce a detector that issufficiently transparent to a radiotherapy beam so that the dose isdelivered to the patient with accuracy and precision, minimising thephenomena of scattering and deterioration of the beam. The constructionof a said detector must also take into account problems of capacity,microphonic effect and mechanical stability.

It is one of the objectives of the present invention to obtain adosimetry device comprising an assembly of ionisation chambers whichenables monitoring of the dose of a beam directed onto a patient, thedevice not having the disadvantages of the prior art devices.

More specifically, the objective of the present invention is to minimisethe water equivalent thickness of a dosimetry device so as to deliver adose to a patient which is the most accurate and precise as possible.

An additional objective of the present invention is to obtain gooddetection dynamics, in particular by eliminating or reducing theintrinsic capacitance of the support plates of the ionisation chamberswhilst reducing the thickness of these support plates.

A further objective of the present invention is to provide a devicewhose collecting electrodes maintain uniform response over their entiresurface by preventing the deformation of these support plates of narrowthickness subjected to a strong electric field.

A further objective of the present invention is to provide a device ableto measure with precision both the dose deposited by a beam and thefield of this same beam.

A further objective of the present invention is to provide a<<universal>> device allowing measurement of the properties of a beamobtained using both a passive delivery technique and a dynamictechnique.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to a devicefor the online monitoring of an ionising beam generated by a radiationsource and delivered to a target, the said device comprising a pluralityof support films arranged in parallel and separated from each other by agap; the said support films being positioned perpendicularly relative tothe central axis of the ionising beam and forming a succession ofionising chambers of which at least one ionising chamber is formed usingsupport films having a thickness of 100 μm or less; each of the supportfilms having on its two surfaces one or more electrodes set at apotential such that the two sides of each of the support films has thesame polarity; the support films being arranged so that the successivesupport films have alternate polarisation; the said device furtherhaving additional means capable of equilibrating the electrostaticforces present inside the said ionisation chamber formed using supportfilms having a thickness equal to or less than 100 μm.

Preferably, in the device of the invention, the at least one ionisationchamber is made using support films having a thickness of less than 20μm, preferably equal to or less than 15 μm, more preferably equal to orless than 10 μm, further preferably equal to or less than 5 μm, stillfurther preferably equal to or less than 1 μm.

Preferably in the device of the invention, the additional means comprisea rigid plate, parallel to and positioned facing the support filmcomprising a collecting electrode on each of its sides, and taking partin the formation of the ionisation chamber made using support filmshaving a thickness equal to or less than 100 μm; the rigid plate furthercomprising at least one electrode placed at a potential capable ofequilibrating the electrostatic forces present inside the ionisationchamber.

Preferably, in the device of the invention, the additional meanscomprise a rigid or flexible plate, preferably flexible, parallel to andpositioned opposite the support film comprising a polarising electrodeon each of its sides, and taking part in the formation of the ionisationchamber prepared using support films having a thickness equal to or lessthan 100 μm; the rigid or flexible plate further comprising at least oneelectrode placed at a potential capable of equilibrating theelectrostatic forces present inside the ionisation chamber.

Preferably, in the device of the invention, the gaps between eachsupport film are constant.

Preferably, in the device of the invention, at least one of the supportfilms having a thickness equal to or less than 100 μm comprises anelectrode at least on one of its surfaces, preferably a collectingelectrode, connected to measuring electronics via a conductive tracelocated on the same side of the support film as the side comprising thesaid electrode, so that the mechanical stability of the said supportfilm is not detrimentally affected.

Preferably, the device of the invention comprises support films havingcollecting electrodes on their two surfaces alternating with supportfilms having polarising electrodes on their two surfaces.

Preferably, in the device of the invention, each collecting electrodeelectrode is connected to measurement electronics by a conductive tracelocated on the same side of the support film as the side comprising thesaid collecting electrode.

Preferably, in the device of the invention, some collecting electrodesassume the shape of strips arranged in parallel.

According to another aspect, the invention concerns a device intended tomeasure ionising beams, the device comprising a support film having twosurfaces and having a thickness equal to or less than 100 μm, preferablyless than 20 μm, more preferably equal to or less than 15 μm, furtherpreferably equal to or less than 10 μm, still further preferably equalto or less than 5 μm, still further preferably equal to or less than 1μm; the support film comprising an electrode on at least one of itssurfaces, preferably a collecting electrode, connected to measurementelectronics by a conductive trace located on the same side of thesupport film as the side comprising the electrode.

Preferably, in the device of the invention, the electrode assumes theshape of a disc whose perimeter is separated by a gap or insulatingresin from a guard layer which extends over the remainder of the supportfilm, and the disc-shaped electrode is connected to measurementelectronics by a trace located on the same side of the said support filmas the side comprising the disc-shaped electrode, the trace being coatedwith an insulating resin, and the insulating resin is coated with a thinlayer of conductive material which extends over the guard layer.

According to another aspect, the invention concerns a method for theonline monitoring of an ionising beam generated by a radiation sourceand delivered onto a target, the method comprising the steps of:

a) providing a plurality of support films arranged in parallel andseparated from each other by a gap; the support films being positionedperpendicularly relative to the central axis of the ionising beam andforming a succession of ionisation chambers of which at least oneionisation chamber is formed using support films having a thicknessequal to or less than 100 μm; each of the support films having one ormore electrodes on its two surfaces;

b) placing each of the support films at a potential such that the twosurfaces of each of the support films has the same polarity;

c) arranging the support films such that the successive support filmshave alternating polarisation;

d) determining the electrostatic forces present inside the ionisationchamber formed by support films having a thickness equal to or less than100 μm;

e) equilibrating the electrostatic forces by means of additional means.

Preferably, in the method of the invention, the at least one ionisationchamber is made using support films having a thickness less than 20 μm,preferably equal to or less than 15 μm, more preferably equal to or lessthan 10 μm, further preferably equal to or less than 5 μm, still furtherpreferably equal to or less than 1 μm.

Preferably, in the method of the invention, at least one of the supportfilms having a thickness equal to or less than 100 μm comprises anelectrode on at least one of its surfaces, preferably a collectingelectrode, connected to measurement electronics by a trace located onthe same side of the support film as the side comprising the saidelectrode, so that the mechanical stability of the said support film isnot detrimentally affected.

Preferably, in the method of the invention, the additional meanscomprise a rigid or flexible plate comprising at least one electrodeplaced at a potential capable of equilibrating the electrostatic forcespresent inside the said ionisation chamber.

Preferably, in the method of the invention, the equilibration stepfurther comprises the application of a suitable voltage to the supportfilms.

According to another aspect, the invention concerns the use of thedevice as described above for online monitoring of beams of particlesobtained using passive delivery techniques.

According to another aspect, the invention concerns the use of thedevice as described above for online monitoring of beams of particlesobtained using dynamic delivery techniques.

BRIEF DESCRIPTION OF THE FIGURES

The following drawings are given for illustration purposes and are notin any way to be construed as limiting the scope of the presentinvention. Also, the proportions of the different figures are not drawnto scale.

FIG. 1 illustrates a first embodiment of the invention comprising one ortwo integral ionisation chambers depending on whether or not one of thesupport films located at the end is flexible or rigid.

FIG. 2 illustrates one surface of a support film comprising a collectingelectrode connected to measurement electronics.

FIG. 3 illustrates one surface of a support film comprising a collectingelectrode that is disc-shaped connected to measurement electronics.

FIG. 4 illustrates a second embodiment of the invention in which all thesupport films are flexible.

FIG. 5 illustrates a third embodiment of the invention comprising twointegral ionisation chambers and two ionisation chambers in strip form.

FIG. 6 illustrates a fourth embodiment of the invention comprising twopairs of integral ionisation chambers and two pairs of strip ionisationchambers.

FIG. 7 illustrates a fifth embodiment of the invention comprisingintegral ionisation chambers, strip ionisation chambers and tworeference ionisation chambers.

FIG. 8 illustrates a sixth embodiment of the invention comprisingintegral ionisation chambers, strip ionisation chambers, referenceionisation chambers and ionisation chambers comprising disc-shapedcollecting electrodes.

FIG. 9 illustrates a seventh embodiment comprising two referenceionisation chambers surrounded by two assemblies of ionisation locatedon each side of these reference ionisation chambers, a first assembly ofionisation chambers comprising strip ionisation chambers and integralionisation chambers, a second assembly comprising strip ionisationchambers and ionisation chambers comprising disc-shaped collectingelectrodes.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the dosimetry device of the present inventioncomprising at least two ionisation chambers including at least twoflexible films supporting one or more electrodes and called <<supportfilms>> 10, 20 made in material of low density with a mean atomic weightof less than 20, having good flexibility and good resistance toradiation, such as biaxially-oriented polyethylene terephthalate betterknown as mylar, or poly(4,4′-oxydiphenylene-pyromellitimide better knownas kapton, these materials not in any way forming a limitation to thepresent invention. Preferably, the at least two support films have athickness of between one micrometre et one millimetre, more preferablybetween one micrometre and one hundred micrometres, further preferablybetween one micrometre and twenty micrometres.

At least two support films 10, 20 forming a first ionisation chamber arecoated on their two surfaces with a layer of conductive material actingas electrode. Preferably, the said conductive material is deposited onthe support film by a depositing technique so as to obtain a layer ofconductive material of between one nanometre and one micron, preferablybetween 100 nanometres and one micron, more preferably between 100 and500 nanometres. Preferably the said conductive material is a metal orgraphite, more preferably a metal.

Compared with known support plates in the state of the art and generallyobtained using the PCB technique, the support films of the presentinvention have the advantage that they produce less scattering anddeterioration of the properties of the beam. Nonetheless the reductionin the thickness of the support films compared with those commonly usedin the state of the art results in the onset of new problems, one firstproblem being the locating of the trace returning the signal to a signalmeasuring device, a second problem being a major capacitive effect atthe films, and a third problem being the vibration of the films whenthey are subjected to an electric potential.

Conventionally, a collecting electrode is connected to a trace by a viapassing through an insulating layer arranged between the surface of theelectrode and the support plate, the said trace returning the signal tomeasuring equipment. For a support film whose thickness it is desired tominimise, this arrangement is not desirable.

FIG. 2 illustrates a support film of the present invention comprising acollecting electrode 11 intended to measure a beam delivered using adynamic technique, this type of electrode being called <<integralcollecting electrode>>, the said collecting electrode 11 being connectedto measuring electronics 9 by a trace 13 located on the same side of thesupport film as the electrode 11. The said trace is deposited on eachsupport film using the same deposition technique as the one used fordepositing the electrodes. Preferably, each collecting electrode and thetrace connecting it to the measuring apparatus is separated from a guardlayer 12 by a vacuum 14 or insulating resin 14 surrounding the perimeterof the collecting electrode. FIG. 3 shows a support film comprising adisc-shaped electrode intended for the measurement of a beam deliveredby a passive technique. Since the trace of this collecting electrodemust not be exposed to the beam otherwise it would provide measurementdependent on the field of this beam, this said trace is coated with athin layer of insulating resin, itself coated with a thin layer ofconductive material extending over the guard layer.

The capacitance of a capacitor is directly proportional to the area ofthe capacitor and inversely proportional to the distance separating theplates of the capacitor. A support film comprising a collectingelectrode on one surface and a polarisation electrode on its othersurface can be likened to a capacitor. For a support film having athickness as in the device of the present invention, with a potentialdifference between the two electrodes located on the two sides of thefilm, the risk of breakdown of the film is very high. The breakdown of afilm is a discharge occurring between the two insulated plates of thecapacitor when too many charges have accumulated on one side of thecapacitor, the discharge damaging the insulating layer of the capacitor.

Also, a major capacitive effect at the support film will result indelaying the transmission of charges towards the measuring electronicsand increasing the detector response time. There is therefore a riskthat detection of the dose deposited by the beam will be initiated atthe time when the necessary dose has already been sent to the patient,and that an excess dose is sent damaging healthy tissues.

In the device shown in FIG. 1, the arrangement of the electrodes on thesupport films solves these capacitance problems. Each support film 10,20 on its two surfaces comprises an electrode having the samepolarisation. A first support film 10 comprises on its two surfaces acollecting electrode 11, 15 whose polarisation is preferably close toearth. The two surfaces of a second support film 20 each comprise apolarisation electrode 21, 22 preferably connected by a trace to agenerator placed at a positive or negative potential. Each conductivetrace connecting a polarisation electrode to the generator is located onthe said side of the support film as the said polarisation electrode. Inthis manner two support films 10, 20 are obtained in which the twosurfaces of one same support film are similarly polarised, which allowsthe capacitive effect to be greatly reduced either side of a supportfilm.

Each support film 10, 20 is held in a support e.g. a support in epoxyresin, the said support guaranteeing good mechanical tensioning and goodinsulation of each support film. The two support films are secured sothat a gap is created therebetween. The support comprises spacers forexample having high electrical resistance, whose dimensions arecalibrated with very small tolerances. The gaps separating the supportfilms must have high guaranteed precision since the field, and hence theelectrostatic force, depend on the applied electric voltage and on thedistance between each support film.

Advantageously, the producing of a detector comprising flexible supportfilms of relatively narrow thickness must also take into account themicrophonic effect. The difference in potential created between twosupport films as thin as those of the present invention has the effectof buckling and/or vibrating these support films, which deteriorates thedetection of the charges created by ionisation of the gas containedbetween the two support films through which a beam passes, since the gapbetween these two support films varies continuously. Similarly, externalnoise also produces a microphonic effect on said ionisation chamber; thedevice must therefore also minimise the contribution made by externalnoise.

To reduce this microphonic effect and more especially to obtain auniform response of the collecting electrode over its entire surface,two plates or films 16, 18 are positioned either side of the ionisationchamber 1 formed by the two support films 10, 20. These two plates orfilms 16, 18 comprise electrodes 17, 19 placed at a potential chosen soas to set up an electrostatic force F_(E2) equilibrating with theelectrostatic force F_(E1) created by polarisation of the support films10, 20 of the ionisation chamber 1.

A first plate 16, preferably rigid, is positioned facing and parallelwith the collecting electrode 15 located towards the outside of theionisation chamber 1. This plate 16 comprises an electrode 17 which isplaced at a potential chosen so as to equilibrate the electrostaticforce F_(E1) applied to the support film 10 and resulting from theelectric field set up by the difference in polarity between thecollecting electrode 11 and the polarisation electrode 21 locatedtowards the inside of the ionisation chamber 1. Preferably, the gapseparating the electrode 17 contained on the first plate 16 from theelectrode 15 contained on the support film 10, is identical to the gapseparating the collecting 11 and polarisation 21 electrodes containedinside the ionisation chamber 1. More preferably, the voltage applied tothe electrode 17 of plate 16 is equal to the voltage applied to thepolarisation electrodes 21, 22 of the support film 20.

A second plate 18, which may or may not be rigid, is positioned facingand parallel with the support film 20 comprising the polarisationelectrodes 21, 22. This second plate 18 comprises an electrode 19 placedat a potential chosen to equilibrate the electric force F_(E1) createdby polarisation of the electrodes 21, 22 of the support plate 20. It isnot necessary for this second plate 18 to be rigid if the electrode 19contained on this plate 18 is not a collecting electrode, this electrode19 together with electrode 22 therefore not forming an ionisationchamber.

Since the support film 10 comprises a collecting electrode 11, 15 on itstwo surfaces, charges created by ionisation of the gas by the beam arecollected on the two sides of this film. Differences in the charges oneach plate of one same film may lead to a slight capacitive effect,possibly interfering with measurement time at the measurementelectronics. To avoid this inconvenience, the electric signal producedat the two collecting electrodes 11, 15 and resulting from ionisation ofthe gas is preferably physically summed before being sent to themeasurement electronics. The support film 10 comprising the twocollecting electrodes 11, 15 located on each side of this same film istherefore common to two ionisation chambers, a first ionisation chamber1 being formed by the two support films 10, 20 and a second ionisationchamber 2 being formed by the support film 10 comprising the collectingelectrodes and the rigid plate 16. It is therefore preferable in thiscase that these said ionisation chambers 1, 2 should have the same gap.This is why the plate 16 located facing the collecting electrode 15 ofthe support film 10 is a rigid plate, thereby reducing microphoniceffects and guaranteeing a constant gap in the two ionisation chambers1, 2 required for exact, precise dose measurement.

FIG. 4 shows one embodiment of the invention in which the rigid plate 16has been replaced by a support film 30 having a polarisation electrodeon its two surfaces, this support film preferably being identical to thesupport film 20 comprising a polarisation electrode on its two surfaces.This gives an assembly of two ionisation chambers 1, 2 comprising acollecting electrode common to these two ionisation chambers andcollecting the same quantity of charges. Two films 18, 40 respectivelycomprise electrodes 19 and 41 preferably placed at identical potentialor close to the potential of the collecting electrodes. These films 18,40 are positioned either side of the said assembly of ionisationchambers and their electrodes create an equilibrating electrostaticforce F_(E2) of opposite direction to the electrostatic forces F_(E1)applied to the support films 10, 30 comprising the polarisationelectrodes placed at a negative potential for example. The films 18, 40located either side of the said assembly of ionisation chambers 1, 2must not necessarily be rigid since no charge is collected in the spaceformed by these films 18, 40 and the opposite-facing support films 20,30.

As in the preceding case, the signals collected on the collectingelectrode of the ionisation chamber 1 and 2 are summed and sent towardsmeasurement electronics e.g. a charge integrator.

FIG. 5 illustrates another embodiment of the present invention dedicatedto the so-called Pencil Beam Scanning technique. The device comprises anassembly of parallel ionisation chambers, each ionisation chambercomprising a flexible, thin support film on which a thin layer ofconductive material is deposited by evaporation process which acts ascollecting or polarisation electrode. Two support films 40, 18 on whichelectrodes are deposited by evaporation deposition are preferablyearthed and positioned parallel either side of the said assembly ofionisation chambers. The assembly of ionisation chambers comprises twosub-assemblies of ionisation chambers. A first sub-assembly ofionisation chambers comprises two integral ionisation chambers 203, 204measuring the dose deposited by the beam. This first sub-assembly ofionisation chambers comprises:

-   -   a first support film 105 comprising a polarisation electrode on        its two surfaces;    -   a second support film 104 comprising a collecting electrode on        its two surfaces, this support film being common to the two        ionisation chambers 203, 204 of the first sub-assembly of        ionisation chambers, the collecting electrode covering at least        90% of the support film, being surrounded by a guard electrode        and whose structure is the one illustrated in FIG. 2;    -   a third support film 103 comprising a polarisation electrode on        its two surfaces, this support film being common with the        ionisation chamber 203 of the first sub-assembly of ionisation        chambers and with one of the ionisation chambers 202 of the        second sub-assembly of ionisation chambers.

The said collecting and polarisation electrodes extend over a regioncovering at least 90% of their support film so as to create and collecta maximum quantity of charges. A second sub-assembly of two ionisationchambers 201, 202 comprises:

-   -   the said support film 103;    -   a second support film 102 on which collecting electrodes are        deposited in the form of strips, surrounded by a guard layer        separated from these electrodes by an insulating material, so as        to measure the beam field, each strip of one surface of the        support film being connected to measurement electronics by a        conductive trace located on the same side of the said second        support film;    -   a third support film 101 comprising a polarisation electrode on        its two surfaces.

The first sub-assembly of ionisation chambers 203, 204 lies adjacent thesecond sub-assembly of ionisation chambers 201, 202, one ionisationchamber 203 of the first sub-assembly having a support film 103 incommon with an ionisation chamber 202 of the second sub-assembly ofionisation chambers. The first sub-assembly of ionisation chamberscomprises two integral ionisation chambers 203, 204 formed by a supportfilm 103, 105 comprising a polarisation electrode on surface side, and asupport film 104 common with the two ionisation chambers 203, 204, thesupport film 104 comprising a collecting electrode on each surface.

Preferably, the assembly of ionisation chambers of the device of thepresent invention comprises a third and a fourth sub-assembly ofionisation chambers as illustrated in FIG. 6. Preferably, the integralionisation chambers 203, 204, 205, 206 are located towards the inside ofthe device whereas the ionisation chambers 201, 202, 207, 208 comprisingelectrodes in the form of strips are located towards the ends of thedevice. With this arrangement it is possible to have a stable precisesignal in the integral ionisation chambers 203, 204, 205, 206 measuringthe dose deposited by the beam. Preferably, a support film whether ornot comprising a collecting electrode and earthed on each side isalternated with a support film comprising a polarisation electrode oneach side. This redundancy of ionisation chambers allows repeat ofmeasurements and ensures that the device functions correctly therebyguaranteeing maximum secure measuring of the dose delivered to thepatient. In the event of breakdown of one of the support films, it isalways possible to control the dose sent to the patient.

FIG. 6 shows two sub-assemblies of two adjacent, integral ionisationchambers 203, 204, 205, 206 in which:

-   -   a support film 104 is common to two ionisation chambers 203, 204        and on its two surfaces it comprises a collecting electrode;    -   a support film 105 is common to two ionisation chambers 204, 205        and each of its two surfaces comprises a polarisation electrode;    -   a support film 106 is common to two ionisation chambers 205, 206        and each of its two surfaces comprises a collecting electrode.

One sub-assembly of two ionisation chambers 201, 202 having in common asupport film 102 comprising collecting electrodes in strip form on eachof its two surfaces. One ionisation chamber 202 of this sub-assembly ispositioned adjacent an integral ionisation chamber 203 and has in commonwith this ionisation chamber 202 a support film 103 comprising apolarisation electrode on each of its two surfaces.

A second sub-assembly of two ionisation chambers 207, 208 has in commona support film 108 comprising collecting electrodes in strip form oneach of its two surfaces. For reasons of clarity, only two measurementelectronic devices connected to the electrodes are illustrated. Oneionisation chamber 207 of this sub-assembly is positioned adjacent anintegral ionisation chamber 206 and has in common with this ionisationchamber 206 a support film 107 comprising a polarisation electrode oneach of its two surfaces. Finally, one support film 18, 40 comprising anelectrode facing the polarisation electrodes positioned towards theoutside of the ionisation chambers 201, 208 that are located at the endsof the assembly of ionisation chambers allows the equilibrating ofelectrical forces due to polarisation of the electrodes 101, 103, 105,107, 109 and contributes towards stabilising the support films of eachionisation chamber of the assembly.

An additional sub-assembly of two ionisation chambers 301, 302 can beinserted in the said assembly of ionisation chambers as illustrated inFIG. 7. Preferably, this sub-assembly of ionisation chambers 301, 302 isarranged in the middle of the device, between the two sub-assemblies ofintegral ionisation chambers 203, 204 and 205, 206. This additionalsub-assembly of ionisation chambers 301, 302 comprises a support film onwhich an electrode is deposited on the two sides of its surface, theseelectrodes equilibrating the electrostatic fields inside the device andable to be used as collecting electrodes to provide a reference signalwhen measuring, in a water phantom, a non-scanned beam for which it isdesired to intercept the entirety of the flow of particles at the timeof measurement in the said phantom. For conventional measurement in awater phantom it is difficult to position a reference chamber in a flowof particles without perturbing the measurement thereof. With one ormore reference chambers in the device, said measurement is no longerperturbed.

Preferably the first sub-assembly of ionisation chambers 201, 202,through which the beam passes and positioned at the input of the device,comprises collecting electrodes in strip form oriented along an axis xorthogonal to the axis of the beam. The last sub-assembly of ionisationchambers 207, 208, through which the beam passes, comprises collectingelectrodes in strip form oriented along an axis y orthogonal to the axisof the beam and to the said axis x.

This device can be placed at the output of a radiation unit and scarcelyperturbs beam properties on account of its low water equivalentthickness, minimising the effects of angular and longitudinalscattering. It is possible for example to calculate the water equivalentthickness of a detector of the present invention by considering the lastexample of FIG. 6 which comprises 13 support films made ofbiaxially-oriented polyethylene terephthalate (mylar) e.g. 2.5 μm thickand coated on the two sides with a thin layer of gold or aluminium ofthickness 200 nm for example, each support film being separated from theother by an air gap of 5 mm for example. The different parameters ofthis present example are reproduced in Table 1 for a beam of 200 MeVpassing through this example of the device.

TABLE 1 1_(mylar) (cm) ρ_(mylar) (g/cm³)$\left( \frac{{1 \cdot d}\; E}{\rho \; {dx}} \right)_{mylar}\left( {{MeV}*{{cm}^{2}/g}} \right)$WET_(mylar) (cm) 2.5E−04 1.397 4.22E−03 2.25E−04 1_(gold) (cm) ρ_(gold)(g/cm³)$\left( \frac{{1 \cdot d}\; E}{\rho \; {dx}} \right)_{gold}\left( {{MeV}*{{cm}^{2}/g}} \right)$WET_(gold) ( cm)   2E−05 19    2.32E−03 1.94E−04 1_(air) (cm) ρ_(air)$\left( \frac{{1 \cdot d}\; E}{\rho \; {dx}} \right)_{air}\left( {{MeV}*{{cm}^{2}/g}} \right)$WET_(air) (cm) 0.5 1.21E−03 3.95E−03 5.20E−04

This example of embodiment of the invention comprises 13 mylar films, 26layers of gold and 12 air gaps. The water equivalent thickness of saiddetector is therefore (13*2, 25E-04)+(26*1, 94E-04)+(12*5, 20E-04)=0.014cm for a detector length of about 6.13 cm. The thicknesses of thedifferent materials are given solely as examples, other thicknesses andother materials possibly being chosen to implement the presentinvention. Similarly, some support films may differ from each other inrespect of thickness and the materials chosen.

A device allowing measurement of the field and dose of a beam obtainedusing a so-called passive delivery technique can be obtained byreproducing the same structure as one of the devices described in thepreceding embodiments, and by replacing the integral ionisation chamberswhose collecting electrodes cover almost all the surface of the supportfilms, by ionisation chambers whose collecting electrodes contained onthe support films are disc-shaped.

FIG. 8 illustrates another embodiment of the present invention allowingboth dosimetry of beams of particles obtained using dynamic techniquesand dosimetry of beams obtained using passive techniques. Thisembodiment illustrated in FIG. 8 comprises both integral ionisationchambers 203, 204, 205, 206 and ionisation chambers 401, 402, 403, 404whose collecting electrodes are disc-shaped. In this embodiment, twosub-assemblies of two integral ionisation chambers and twosub-assemblies of tow ionisation chambers with disc-shaped collectingelectrodes are arranged towards the middle of the device, for examplesymmetrically relative to an assembly of two reference ionisationchambers 301, 302. Said device may comprise an assembly of fourteenionisation chambers also counting ionisation chambers 201, 202, 207, 208which comprise electrodes in the form of strips. The device alsocomprises two support films 18, 40 positioned either side of thisassembly of ionisation chambers and allowing equilibration ofelectrostatic forces and stabilization of the distances between eachsupport film.

To reduce the number of ionisation chambers and support plates, whilstmaintaining the redundancy characteristics of the device and thepossibility of measuring beams obtained both with dynamic and passivedelivery methods, each collecting electrode contained on a support filmof an integral ionisation chamber and of an ionisation chamber withelectrode of reduced size is connected to its own measurementelectronics. One embodiment of the present invention is illustrated inFIG. 9 and comprises:

-   -   two first ionisation chambers 201, 202 comprising collecting        electrodes in strip form, these ionisation chambers being formed        by:        -   a first support film 101 comprising a polarisation electrode            on its two surfaces, each electrode being connected to a            voltage generator HV2;        -   a second support film 102 positioned facing the first            support film 101 and comprising collecting electrodes in            strip form on its two surfaces, arranged in identical manner            on the two surfaces, each strip of one surface and each            strip on the other side of the surface of the support film            being connected to one same measurement electronics;        -   a third support film 103 positioned facing the second            support film 102 and comprising a polarisation electrode on            its two surfaces, each electrode being connected to a            voltage generator HV2;    -   A third ionisation chamber 501 formed by:        -   the said third support film 103 and;        -   a fourth support film 119 positioned facing the third            support film 103 and comprising on the side facing the            support film 103 an integral collecting electrode connected            to its own measurement electronics;    -   A fourth ionisation chamber 502 formed by:        -   a fifth support film 120 positioned facing the fourth            support film 119 and comprising on its two surfaces a            polarisation electrode connected to a voltage generator HV3;        -   the fourth support film 119 comprising on the side facing            the fifth support film 120 an integral collecting electrode            connected to its own measurement electronics;    -   A fifth and a sixth reference ionisation chamber 301, 302 formed        by:        -   the said fifth support film 120;        -   a sixth support film 111 positioned facing the fifth support            film 120 and comprising a collecting electrode on its two            surfaces;        -   a seventh support film 121 positioned facing the sixth            support film 111 and comprising on its two surfaces a            polarisation electrode connected to a high voltage generator            HV2;    -   A seventh ionisation chamber 503, formed by:        -   the said seventh support film 121;        -   an eighth support film 122 positioned facing the seventh            support film 121 and comprising a disc-shaped collecting            electrode surrounded by a guard, the electrode being            connected to its own measurement electronics by a trace            coated with an insulating resin, the electrode facing the            said seventh support film;    -   An eighth ionisation chamber 504 formed by:        -   a ninth support film 123 positioned facing the eighth            support film 122 an comprising a polarisation electrode on            its two surfaces;        -   the said eighth support film 122 comprising a disc-shaped            collecting electrode, surrounded by a guard, the electrode            being connected its own measurement electronics by a trace            coated with an insulating resin, the electrode facing the            said ninth support film;    -   A ninth and a tenth ionisation chamber 207, 208 comprising        electrodes in strip form, these ionisation chambers being formed        by:        -   the said ninth support film 123 comprising a polarisation            electrode on its two surfaces, each electrode being            connected to a voltage generator HV3;        -   a tenth support film 108 positioned facing the ninth support            film 123 and comprising on its two surfaces collecting            electrodes in strip from arranged in identical manner on the            two surfaces, each strip of one surface of the support film            and its opposite facing strip on the other side of the            surface of the support film being connected to one same            measurement electronics;        -   an eleventh support film 109 positioned facing the tenth            support film 108 and comprising a polarisation electrode on            its two surfaces, each electrode being connected to a            voltage generator HV3.            The assembly of these ionisation chambers is contained            between two support films 40, 18 each comprising an            electrode previously placed at the same potential as the            collecting electrodes and positioned facing the support            films of the first and tenth ionisation chamber.

This embodiment therefore overall comprises thirteen support plates andhave a water equivalent thickness of 0.014 cm for device measuring about6 cm and able to be used for measuring the dose and field of differenttypes of beam. Although a single high voltage generator is sufficient topolarise all the polarisation electrodes, this embodiment of the presentinvention comprises two high voltage generators HV2, HV3 connected tothe polarisation electrodes in the manner described above, in order tohave redundancy of the ionisation chambers and to ensure measurement ofthe dose in the event of a problem with one of the two generators or inthe event of breakdown of one of the support films comprising apolarisation electrode.

1. A device for the online monitoring of an ionising beam generated by aradiation source and delivered onto a target, the device comprising aplurality of support films arranged in parallel and separated from eachother by a gap; the support films being positioned perpendicularlyrelative to the central axis of the ionising beam and forming asuccession of ionisation chambers of which at least one ionisationchamber is formed using support films having a thickness equal to orless than 100 μm; each of the support films having on its two surfacesone or more electrodes set at a potential such that the two surfaces ofeach of the support films have the same polarity; the support filmsbeing arranged such that the successive support films have alternatingpolarisation; the device further having an additional componentconfigured to equilibrate the electrostatic forces present inside theionisation chamber formed using support films having a thickness equalto or less than 100 μm.
 2. The device according to claim 1, wherein theat least one ionisation chamber is formed using support films having athickness of less than 20 μm.
 3. The device according to claim 1,wherein the additional component configured to equilibrate theelectrostatic forces comprises a rigid plate, parallel to and facing thesupport film comprising a collecting electrode on each of its surfaces,and taking part in the formation of the ionisation chamber formed usingsupport films having a thickness equal to or less than 100 μm; the rigidplate further comprising at least one electrode set at a potentialcapable of equilibrating the electrostatic forces present inside theionisation chamber.
 4. The device according to claim 1, wherein theadditional component configured to equilibrate the electrostatic forcescomprises a rigid or flexible plate parallel to and facing the supportfilm comprising a polarisation electrode on each of its surfaces, andtaking part in the formation of the ionisation chamber formed usingsupport films having a thickness equal to or less than 100 μm; the rigidor flexible plate further comprising at least one electrode set at apotential capable of equilibrating the electrostatic forces presentinside the ionisation chamber.
 5. The device according to claim 1,wherein the gaps between each support film are constant.
 6. The deviceaccording to claim 1, wherein at least one of the support films having athickness equal to or less than 100 μm comprises an electrode on atleast one of its surfaces.
 7. The device according to claim 1 comprisingsupport films having collecting electrodes on their two surfacesalternating with support films having polarisation electrodes on theirtwo surfaces.
 8. The device according to claim 7, wherein eachcollecting electrode is connected to measurement electronics by a tracelocated on the same side of the support film as the side comprising thecollecting electrode.
 9. The device according to claim 1 wherein somecollecting electrodes assume the shape of strips arranged in parallel.10. A device for measuring ionising beams, the device comprising asupport film having two surfaces and having a thickness equal to or lessthan 100 μm, the support film comprising an electrode on at least onethe surfaces.
 11. The device according to claim 9, wherein the electrodeis disc-shaped whose perimeter is separated by a gap or insulating resinfrom a guard layer which extends over the remainder of the support film,and wherein the disc-shaped electrode is connected to measurementelectronics by a trace located on the same side of the support film asthe side comprising the disc-shaped electrode, the trace being coatedwith an insulating resin, and the said insulating resin coated with athin layer of conductive material which extends over the guard layer.12. A method for online monitoring of an ionising beam generated by aradiation source and delivered to a target, the method comprising:providing a plurality of support films arranged in parallel andseparated from each other by a gap; the support films being positionedperpendicularly relative to the central axis of the ionising beam andforming a succession of ionisation chambers of which at least oneionisation chamber is formed using support films having a thicknessequal to or less than 100 μm; each of the support films having one ormore electrodes on its two surfaces; setting each of the support filmsat a potential such that the two surfaces of each of the support filmshave the same polarity; arranging the support films such that thesuccessive support films have alternating polarisation; determining theelectrostatic forces present inside the ionisation chamber formed usingsupport films having a thickness equal to or less than 100 μm; and c)equilibrating the electrostatic forces.
 13. The method according toclaim 12, wherein the at least one ionisation chamber is formed usingsupport films having a thickness less than 20 μm.
 14. The methodaccording to claim 12, wherein at least one of the support films havinga thickness equal to or less than 100 μm comprises an electrode at leaston one of its surfaces.
 15. The method according to claim 12, whereinequilibrating the electrostatic forces is performed by a rigid orflexible plate comprising at least one electrode set at a potentialcapable of equilibrating the electrostatic forces present inside theionisation chamber.
 16. The method according to claim 12, wherein theequilibrating step further comprises applying a suitable voltage to thesupport films.
 17. A method for online monitoring beams of particlesdelivered using passive delivery techniques, the method comprisingutilizing the device according to claim
 1. 18. A method for onlinemonitoring beams of particles delivered using dynamic deliverytechniques, the method comprising utilizing the device according toclaim
 1. 19. The device according to claim 6, wherein the electrode is acollecting electrode connected to measurement electronics by a tracelocated on the same side of the support film as the side comprising theelectrode.
 20. The device according to claim 10, wherein the electrodeis a collecting electrode connected to measurement electronics by atrace located on the same side of the support film as the sidecomprising the electrode.